The WS-BPEL specification defines several standard faults. If one of these faults is encountered during simulation, the simulation terminates and shows a red X next to the activity where the fault occurred. Also, you see a listing for the fault in the Console. An example is:
Assign AssignYestoAccept:
Completed with fault: mismatchedAssignmentFailure :
</process/flow/assign[@name='AssignYestoAccept']>